Current derivative sensor

ABSTRACT

A system and method for detecting, measuring, and reporting a time derivate of a current signal (di/dt) is provided. A sensing element detects current from a load. The sensing element includes an inductor. The inductor is located in series with the load and includes associated parasitic resistance. A differential potential develops across the inductor and the parasitic resistance. The differential potential is amplified and converted to a single-ended value. The single-ended value is then fed to an analog to digital converter that provides an output representative of di/dt.

RELATED APPLICATION

[0001] This application claims priority to and incorporates by referenceU.S. Provisional Application Ser. No. 60/311,653 filed Aug. 10, 2001.

FIELD

[0002] The present invention relates generally to current sensors, andmore particularly, relates to a current derivative sensor.

BACKGROUND

[0003] The ability to detect, measure, and record a rate of currentchange may be critical in high-speed electronic applications. The rateof current change may be referred to as a slope of a current signal, oralternatively, as a time derivative of a current signal (di/dt). Therate of the current change may be important when accounting for unwantednoise, such as electromagnetic interference (EMI) and radio-frequencyinterference (RFI), generated by high-speed circuits. However, mostcurrent sensors detect and measure only the magnitude of the current,and not the rate of current change.

[0004] Often the slope of the current signal is determined usingcomputer simulation techniques, such as finite element analysis, lumpedelement simulation, and behavioral modeling. These computer techniquescan become computationally intensive and include limiting assumptions,which may reduce the accuracy of the simulation result. Therefore, itwould be beneficial to make a direct measurement of di/dt using acurrent derivative sensor.

SUMMARY

[0005] A current derivative sensor and a method for detecting,measuring, and recording a time derivative of a current signal (di/dt)are provided. A sensing element detects current. The current flowsthrough the sensing element, generating a differential potential acrossthe sensing element. A gain circuit amplifies and converts thedifferential potential to a single-ended output. An analog to digitalconverter converts the single-ended output and provides an outputrepresentative of di/dt.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Presently preferred embodiments are described below inconjunction with the appended drawing figures, wherein like referencenumerals refer to like elements in the various figures, and wherein:

[0007]FIG. 1 is a circuit diagram of a current derivative sensor,according to an exemplary embodiment;

[0008]FIG. 2A is a circuit diagram of a current derivative sensor,according to another exemplary embodiment;

[0009]FIG. 2B is a circuit diagram of a current derivative sensor,according to another exemplary embodiment;

[0010]FIG. 3A is a circuit diagram of a calibration circuit, accordingto an exemplary embodiment;

[0011]FIG. 3B is a circuit diagram of a calibration circuit, accordingto another exemplary embodiment;

[0012]FIG. 4 is a graph of selectivity of the sensor, according to anexemplary embodiment; and

[0013]FIG. 5 is a flow chart diagram of a method of measuring a timederivative of a current signal (di/dt), according to an exemplaryembodiment.

DETAILED DESCRIPTION

[0014]FIG. 1 is a circuit diagram of a current derivative sensor 100,according to an exemplary embodiment. The current derivative sensor 100may include a sensing element 110, a gain circuit 118, an analog todigital (AID) converter 120, and a calibration circuit 122. The sensor100 may be a standalone device that can be inserted into a desiredcircuit to make measurements, or it may be fabricated on a die andinterconnected to circuits on a separate die, or may be fabricated onthe same die as the circuit being measured.

[0015] The current derivative sensor 100 may be designed to monitorcurrent 102. The current 102 may be composed of both a DC component anda transient component. The current 102 may be generated by a load 104.The load 104 may be any conductor that can generate a transient currentsignal, such as integrated circuit interconnect metallization,integrated circuit polysilicon, silicided silicon connectors, printedcircuit board traces, insulated wires, and non-insulated wires. For thesake of simplicity, the load 104 is depicted in FIG. 1 as a parallelcombination of a capacitor 106 and a variable resistor 108.

[0016] The current 102 may be detected by the sensing element 110. Thesensing element 110 preferably includes an inductor 112 placed in serieswith the load 104. A parasitic resistance may be associated with theinductor 112. The parasitic resistance is depicted in FIG. 1 as aresistor 114 located in series with the inductor 112. Other devices mayalso be included within the sensing element 110.

[0017] The current 102 may cause a differential potential 116 to developacross the series combination of the inductor 112 and the resistor 114between the electrical nodes labeled in FIG. 1 as V_(positive) andV_(negative). The differential potential 116 may be generated fromsubstantially three origins. A first origin may be a DC potentialproduced when the current 102 flows through the resistor 114. A secondorigin may be a transient potential produced when the current 102 flowsthrough the resistor 114. A third origin may be transient potentialproduced when the current 102 flows through the inductor 112. Thedifferential potential 116 may be defined by the following equation:

v(t)=(I+i(t))R+L(di/dt)   Equation 1

[0018] where: v(t) is the differential potential 116;

[0019] I is the DC current flowing through the resistor 114;

[0020] i(t) is the transient current flowing through the resistor 114;

[0021] R is the resistance from resistor 114;

[0022] L is the inductance from inductor 112; and

[0023] di/dt is the transient current flowing through the inductor 112.

[0024] The current derivative sensor 100 may be designed to measuresubstantially the third origin of the differential potential 116, whichis the transient current di/dt flowing through the inductor 112.

[0025] The calibration circuit 122 may be used to calibrate the currentderivative sensor 100. The calibration circuit 122 may be used tomeasure the parasitic resistance. Additionally, the calibration circuit122 may be used to determine an accurate value of inductance of theinductor 112, which may be needed to correlate the differentialpotential 116 to the magnitude of the di/dt event. The details of thecalibration circuit 122 are discussed below.

[0026] The gain circuit 118 may be used to amplify and convert thedifferential potential 116 to a single-ended output. The gain circuit118 may be a differential-input, single-ended output operationalamplifier (op amp). The addition of single-ended gain circuits 202located at an output of the op amp may be beneficial for amplifyingsmall differential potentials, as shown in FIG. 2A. Alternatively,additional differential-input, single-ended output gain circuits 204located at the output of the op amp may be used to provide additionalamplification for small differential potentials, as shown in FIG. 2B.

[0027] Referring back to FIG. 1, an output of the gain circuit 118 maybe connected to the A/D converter 120, one embodiment of which is shownas a plurality of switches. While Schmitt triggers are used in apreferred embodiment, other switching devices or combination of devicesthat can be triggered may also be employed.

[0028] The Schmitt triggers may be configured such that LOW-to-HIGHinput transition voltages are monotonically increasing fromsubstantially a ground potential to a maximum supply voltage. TheLOW-to-HIGH input transition voltage may be a value of voltage thatcauses a switch to change states from off to on. Alternatively, theLOW-to-HIGH input transition voltages may be monotonically increasingfor a range of voltages located between the ground potential and themaximum supply voltage.

[0029] Additionally, the Schmitt triggers may be configured such thatHIGH-to-LOW input transition voltages are substantially at the maximumsupply voltage. The HIGH-to-LOW input transition voltage may be a valueof voltage that causes a switch to change states from on to off.

[0030] In further alternative embodiments, the A/D converter 120 may beimplemented using a series of voltage comparators having differentreference voltages. Other suitable alternative analog to digitalconversion techniques and circuits may also be used.

[0031] When a small di/dt event occurs, only the Schmitt triggers with aLOW-to-HIGH input transition voltage near the ground potential maychange to a HIGH output. A larger di/dt event may cause an increasingnumber of the Schmitt triggers to change to a HIGH output. A change instate to a HIGH output may be maintained until the A/D converter 120 isreset.

[0032] Before being reset, the output of the A/D converter 120 may bedetected providing an actual measurement of di/dt. The output of the A/Dconverter 120 may be displayed on a thermometer-type scale readout. (Thethermometer-type scale readout is not shown in FIG. 1.) For example, ifnone of the switches have changed to a HIGH output, then the readout maybe substantially zero or at the bottom of the scale. As the number ofswitches that have changed to a HIGH output increases, the readout maybe increased accordingly up the scale. When all of the switches havechanged to a HIGH output, the readout may be substantially at fullscale. This scale readout may be recorded. While the thermometer-typescale readout is used in a preferred embodiment, other methods ofdisplaying the output of the A/D converter 120 may also be employed.

[0033] In an alternative embodiment, the switch values may be convertedto a binary number representing the number of switches in the HIGHstate. For example, if there are seven switches, and a given output ofgain circuit 118 causes the first five switches to change state (e.g.,to a HIGH output from a LOW output), then the switch outputs may beconverter by an encoder circuit (not shown) into the binary value “101”.A suitable encoder circuit may be implemented using a digital counter tocount the number of switch outputs in the HIGH state, a look-up table, acombinational logic circuit, etc.

[0034] Alternatively, the output of the A/D converter 120 may correspondto a memory address, such as a Read-Only-Memory (ROM) address. For thisexample, “101” may be stored in a ROM address corresponding to the firstfive switches being in the HIGH state.

[0035]FIG. 3A is a circuit diagram of a calibration circuit 300,according to an exemplary embodiment. The calibration circuit 300 issubstantially the same as the calibration circuit 122 as shown inFIG. 1. The calibration circuit 300 includes a plurality of precisionmatched current sources 302 of substantially the same DC magnitude, I.The plurality of current sources 302 may be connected in parallelthrough independently controlled switches 304. FIG. 3A depicts theswitches as field-effect transistors; however, other switches may beused. The independently controlled switches 304 may be controlled by avariety of devices, such as a microcontroller.

[0036] Activating a single leg of the parallel network of currentsources 302 by closing one or more of the switches 304 may generate aknown value of DC current when the current through the inductor 112 hasreached steady state. Using the known value of DC current, adifferential potential 116 may develop across resistor 114 and beamplified by the gain circuit 118. The AID converter 120 may detect theamplified differential potential signal. The output of the A/D converter120 may be used to determine the value of the parasitic resistance,depicted in FIG. 1 as resistor 114.

[0037] Using the value of the parasitic resistance, the calibrationcircuit 300 may determine an accurate value of inductance of inductor112, which may be needed to correlate the differential potential 116 tothe magnitude of the di/dt event. A current with a known di/dt may begenerated by incrementally activating successive legs of the parallelnetwork of current sources 302 by closing the switches 304 one at atime. With each successive activation, the current generated may beincreased by substantially the DC magnitude, I, of the current sources302. Because the calibration circuit 300 is able to account for thecontribution of the parasitic resistance, the circuit may accuratelydetermine, or measure, the inductance. As seen with reference toEquation 1, the inductance is equal to the difference between thedifferential potential 116 and the contribution of the parasiticresistance, divided by the known di/dt.

[0038] A clock circuit or a timer may be used to control the successiveactivation of the legs of the parallel network of current sources 302.(The clock circuit and timer are not shown in FIG. 3A.) As shown in FIG.3B, a filter 306 may be added to the calibration circuit 300, as thecurrent waveform generated will have a staircase response. For example,the filter 306 may be a low pass filter. The addition of the low passfilter 306 may smooth the waveform, which may be a closer approximationof a ramp with a constant di/dt. Alternatively, because the rampresponse ideally contains only odd-order harmonics, a filter 306operable to remove even-order harmonics may be added to the calibrationcircuit 300.

[0039] In an alternative embodiment, the calibration circuit 300 mayinclude a control circuit. (The control circuit is not shown in FIG. 1.)The control circuit may include a microcontroller. Alternatively, thecontrol circuit may include a logic circuit providing combinationaland/or sequential logic. For example, the logic circuit may be a statemachine.

[0040] The control circuit may be operable to provide the known value ofDC current by controlling the operation of independently controlledswitches 304. The control circuit may open and close switches 304 basedon what type of calibration is being performed.

[0041] In addition, the control circuit may receive voltage informationfrom the output of the gain circuit 118 and/or the A/D converter 120.The control circuit may receive the voltage information for various DCcurrent values. For example, the control circuit may receive the voltageinformation for different combinations of switches 304 being opened andclosed. The control circuit may store the voltage information for thevarious DC current values.

[0042] When the current derivative sensor 100 is operating, the controlcircuit may receive the voltage information from the output of the gaincircuit 118 and/or the A/D converter 120 and subtract the previouslystored voltage information for the corresponding DC level. As such, thecontrol circuit may provide an output signal that has been compensatedfor the DC component of current.

[0043] Additionally, the control circuit may provide an offset valuebased on the DC component of current to the gain circuit 118.Furthermore, for the A/D embodiment using voltage comparators, thecontrol circuit may provide the different reference voltages to thevoltage comparators. The different reference voltages may or may not belinearly spaced.

[0044] To maximize the sensitivity of the current derivative sensor 100,the inductive component of the differential potential 116 may beemphasized, minimizing the resistive component. Referring back toEquation 1, to emphasize the inductive component, the following equationholds true.

L(di/dt)>>(I+i(t))R   Equation 2

[0045] The inductive component is related to the quality factor Q of theinductor. The quality factor can be defined as:

Q=ωL/R=2πfL/R   Equation 3

[0046] where: ω is the angular frequency of an AC signal and f is theequivalent frequency in Hertz. If the AC signal is represented by onlyits fundamental frequency then:

di/dt=2f.   Equation 4

[0047] Combining Equations 2, 3, and 4 yields the following designequation:

Q>>π(1+I/i).   Equation 5

[0048] The significance of the design equation, Equation 5, on theperformance of the sensor is shown graphically in FIG. 4. FIG. 4 depictsthe selectivity of the current derivative sensor 100. The selectivity ofthe current derivative sensor 100 may be defined as the percentage ofthe differential potential 116 due to the inductive contribution,L(di/dt).

[0049] A ratio is defined between the DC current I and the transientcurrent i. A high ratio may imply that the total current 102 is nearlyconstant with small transient variations. In this situation, theinductive contribution would not be emphasized, and the selectivitywould be low. This may cause the scaled output to be largely a result ofthe parasitic resistance. On the other hand, a low ratio may imply thatthe current 102 is dominated by the transient current i.

[0050] The impact of the quality factor is also depicted in FIG. 4. Alarger value of Q results in a higher selectivity of the inductivecontribution. A high value of Q may be indicative of either a high valueof di/dt or a low value of resistance 114. Conversely, a low value of Qmay be indicative of either a low value of di/dt or a high value ofresistance 114.

[0051]FIG. 5 depicts a flow chart diagram of a method 500 of measuring atime derivative of a current signal (di/dt). Step 502 is sensing thecurrent. The sensing element 110 may be used to sense the current 102generated by the load 104. The differential potential 116 may begenerated as the current 102 flows through the sensing element 110.

[0052] Step 504 is amplifying the differential potential 116. The gaincircuit 118 may be used to amplify the differential potential 116. Thegain circuit 118 may include additional gain circuits 202, 204 toprovide more amplification for small differential potentials. In analternative embodiment, results from the calibration circuit 300, suchas the inductance value, may be used to set the gain of the gain circuit118.

[0053] Step 506 is converting the differential potential 116 to asingled-ended output. This step may be accomplished using the same gaincircuit 118 used for amplification in Step 504.

[0054] Step 508 is triggering the A/D converter. The A/D converter maybe a plurality of Schmitt triggers configured such that the inputtransition voltage operable to trigger a switch from off to on ismonotonically increasing from switch to switch. Only the switches withtransition voltages at or below the voltage applied by the gain circuit118 will turn on. The number of switches that turn on for a given periodof time is representative of the time derivative of the current signal102. The greater the value of voltage at the output of the gain circuit118, the greater the number of switches that will turn on, whichrepresents a greater rate of current change.

[0055] The output of the A/D converter 120 may be displayed on athermometer-type scale readout, which may then be recorded. In analternative embodiment, the switch values may be converted to a binarynumber representing the number of switches in the HIGH state aspreviously discussed.

[0056] The current derivative sensor 100 may be able to detectelectromagnetic interference and/or radio-frequency interference. Forexample, the current derivative sensor 100 may be able to detect di/dtevents ranging from 10³ amps/second (or 1 amp/millisecond) to 10¹²amps/second (or 1 amp/picosecond). However, other results may bepossible based on the effects of the parasitic resistance depicted inFIG. 1 as resistor 114, the quality factor of the inductor 112, and alimit on the gain circuit 118, which may be imposed by the maximumsupply voltage. The ability to measure such a wide range of di/dt eventsmakes the current derivative sensor 100 ideally suited for applicationsinvolving integrated circuits, printed circuit boards, insulated andnon-insulated wiring, and other electrical conductors.

[0057] It should be understood that the illustrated embodiments areexemplary only and should not be taken as limiting the scope of thepresent invention. The claims should not be read as limited to thedescribed order or elements unless stated to that effect. Therefore, allembodiments that come within the scope and spirit of the followingclaims and equivalents thereto are claimed as the invention.

We claim:
 1. A current derivative sensor, comprising in combination: asensing element operable to detect current, wherein the current flowsthrough the sensing element, thereby generating a differential potentialacross the sensing element; and an analog to digital converter providingan output representative of a time derivative of a current signal(di/dt).
 2. The current derivative sensor of claim 1, further comprisinga gain circuit operable to amplify and convert the differentialpotential to a single-ended output.
 3. The current derivative sensor ofclaim 2, wherein the gain circuit is an operational amplifier.
 4. Thecurrent derivative sensor of claim 3, wherein the gain circuit includesa single-ended gain circuit located at an output of the operationalamplifier.
 5. The current derivative sensor of claim 3, wherein the gaincircuit includes a second differential-input, single-ended gain circuitlocated at an output of the operational amplifier.
 6. The currentderivative sensor of claim 1, wherein a load generates the current. 7.The current derivative sensor of claim 6, wherein the load is aconductor operable to generate a transient current signal.
 8. Thecurrent derivative sensor of claim 6, wherein the load is a conductorselected from the group consisting of integrated circuit interconnectmetallization, integrated circuit polysilicon, silicided siliconconnectors, printed circuit board traces, insulated wires, andnon-insulated wires.
 9. The current derivative sensor of claim 1,wherein the sensing element is an inductor located in series with aload.
 10. The current derivative sensor of claim 9, wherein parasiticresistance is associated with the inductor.
 11. The current derivativesensor of claim 1, wherein the differential potential consists of a DCpotential produced as the current flows through parasitic resistancelocated in the sensing element, a transient potential produced as thecurrent flows through the parasitic resistance, and a transientpotential produced as the current flows through an inductor in thesensing element.
 12. The current derivative sensor of claim 1, furthercomprising a calibration circuit operable to determine a parasiticresistance value.
 13. The current derivative sensor of claim 12, whereinthe calibration circuit determines an inductive value using theparasitic resistance value.
 14. The current derivative sensor of claim12, wherein the calibration circuit includes a plurality of precisionmatched current sources connected in parallel through independentlycontrolled switches.
 15. The current derivative sensor of claim 14,wherein the plurality of precision matched current sources providessubstantially identical current magnitudes.
 16. The current derivativesensor of claim 14, wherein the independently controlled switches arefield-effect transistors.
 17. The current derivative sensor of claim 1,wherein the analog to digital converter includes a plurality of Schmitttriggers.
 18. The current derivative sensor of claim 17, wherein theplurality of Schmitt triggers are configured with LOW-to-HIGH inputtransition voltages that are monotonically increasing from substantiallya ground potential to a maximum supply voltage.
 19. The currentderivative sensor of claim 17, wherein the plurality of Schmitt triggersare configured with HIGH-to-LOW input transition voltages that aresubstantially at maximum supply voltage.
 20. The current derivativesensor of claim 17, wherein the plurality of Schmitt triggers stay onuntil reset.
 21. The current derivative sensor of claim 1, wherein theoutput is displayed on a thermometer-type scale readout.
 22. A currentderivative sensor, comprising in combination: an inductor operable todetect current, wherein the current flows through the inductor, therebygenerating a differential potential across the inductor; an operationalamplifier operable to amplify and convert the differential potential toa single-ended output; and a plurality of Schmitt triggers configured toswitch from off to on at monotonically increasing transition voltages,wherein the plurality of Schmitt triggers provide an outputrepresentative of a time derivative of a current signal (di/dt).
 23. Thecurrent derivative sensor of claim 22, wherein a single-ended gaincircuit is located at an output of the operational amplifier.
 24. Thecurrent derivative sensor of claim 22, wherein a differential-input,single-ended gain circuit is located at an output of the operationalamplifier.
 25. The current derivative sensor of claim 22, wherein a loadgenerates the current.
 26. The current derivative sensor of claim 25,wherein the load is a conductor operable to generate a transient currentsignal.
 27. The current derivative sensor of claim 25, wherein the loadis a conductor selected from the group consisting of integrated circuitinterconnect metallization, integrated circuit polysilicon, silicidedsilicon connectors, printed circuit board traces, insulated wires, andnon-insulated wires.
 28. The current derivative sensor of claim 22,wherein the inductor is located in series with a load.
 29. The currentderivative sensor of claim 22, wherein parasitic resistance isassociated with the inductor.
 30. The current derivative sensor of claim22, wherein the differential potential consists of a DC potentialproduced as the current flows through parasitic resistance located inthe sensing element, a transient potential produced as the current flowsthrough the parasitic resistance, and a transient potential produced asthe current flows through an inductor in the sensing element.
 31. Thecurrent derivative sensor of claim 22, further comprising a calibrationcircuit operable to determine a parasitic resistance value.
 32. Thecurrent derivative sensor of claim 31, wherein the calibration circuitdetermines an inductive value using the parasitic resistance value. 33.The current derivative sensor of claim 31, wherein the calibrationcircuit includes a plurality of precision matched current sourcesconnected in parallel through independently controlled switches.
 34. Thecurrent derivative sensor of claim 33, wherein the plurality ofprecision matched current sources provide substantially identicalcurrent magnitudes.
 35. The current derivative sensor of claim 33,wherein the independently controlled switches are field-effecttransistors.
 36. The current derivative sensor of claim 22, wherein theplurality of Schmitt triggers are configured with LOW-to-HIGH inputtransition voltages that are monotonically increasing from substantiallya ground potential to a maximum supply voltage.
 37. The currentderivative sensor of claim 22, wherein the plurality of Schmitt triggersare configured with HIGH-to-LOW input transition voltages that aresubstantially at maximum supply voltage.
 38. The current derivativesensor of claim 22, wherein the plurality of Schmitt triggers stay onuntil reset.
 39. The current derivative sensor of claim 22, wherein theoutput is displayed on a thermometer-type scale readout.
 40. A method ofmeasuring a time derivative of a current signal (di/dt), comprising incombination: sensing current from a load, thereby generating adifferential potential; amplifying the differential potential;converting the differential potential to a single-ended output; andtriggering an analog to digital converter based on the differentialpotential, thereby providing an output representative of a timederivative of a current signal (di/dt).
 41. The method of claim 40,wherein the step of sensing current from a load, the differentialpotential is generated across a sensing element.
 42. The method of claim41, wherein the sensing element is an inductor located in series withthe load.
 43. The method claim 42, wherein parasitic resistance isassociated with the inductor.
 44. The method of claim 40, wherein thedifferential potential consists of a DC potential produced as thecurrent flows through parasitic resistance located in the sensingelement, a transient potential produced as the current flows through theparasitic resistance, and a transient potential produced as the currentflows through an inductor in the sensing element.
 45. The method ofclaim 40, wherein the load is a conductor operable to generate atransient current signal.
 46. The method of claim 40, wherein the loadis a conductor selected from the group consisting of integrated circuitinterconnect metallization, integrated circuit polysilicon, silicidedsilicon connectors, printed circuit board traces, insulated wires, andnon-insulated wires.
 47. The method of claim 40, wherein adifferential-input, single-ended output gain circuit is operable toamplify and convert the differential potential.
 48. The method of claim40, wherein the analog to digital converter includes a plurality ofSchmitt triggers.
 49. The method of claim 40, further comprisingdisplaying an output of the analog to digital converter on athermometer-type scale readout.